Frequency-responsive network



April 13, 1943. 2 l.. F. CURTIS FREQUENCY RESPONSIVE NETWORK Filed Jan. 16, 1942 \S ORNEY Patented Apr. 13, l1943 UNITED 2,316,317 FREQUENCY-RESPONSIVE NETWORK Leslie F. Curtis, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Dela- Ware Application January 16, 1942, Serial No. 426,943 8 Claims. (Cl. Z50-27) This invention relates tc an improved frequency-responsive network and, more particularly, to such a network adapted to be employed in a control system of a modulated-carrier signal-translating apparatus which has a relatively limited frequency-response range but a relatively high sensitivity over the range for the purpose of obtaining a control eect. As used throughout the specification, the terms relatively limited frequency-response range and limited frequency range refer to a range of frequencies which is narrow relative to the mean frequency of Y the range.

One type of frequency-responsive network which has previously been used for such purpose utilizes the principle of rectifying separately the sum and the difference of the voltages existing across the primary and secondary windings of a double-tuned circuit and dierentially combining the rectified voltages so obtained to provide a voltage varyinglin magnitude and polaritywith deviation of the frequency of the signal input to the system from its mean value. A disadvantage of such an arrangement is that with usual design of signal-translating and control circuits and at usual signal frequencies, the magnitude of the output of each of the rectiers varies quite gradually and over a considerable frequency range. Thus, the difference of the rectified voltages also varies gradually with respect to frequency, decreasing the sensitivity ofthe system, and varies between maximum and minimum values which are relatively remotely separated, permitting the control to extend over adjacent signal channels, effectively blanking themv out.

While in certain prior art frequency-responsive networks the sensitivity has been increased, this has been done by decreasing the frequency separationof the maximum and minimum values of the rectified output and also, generally, by reducing the magnitude of the control voltage derived in the network, thus reducing the pull-in" range of the system, which may be undesirable ln certainapplications. In a signal-transmitting system comprising automatic frequency control.

for example, it is highly desirable that the fre-' quency-discriminating network be capable of providing a very large pull-in eifect so that when the oscillator is energized, preparatory to' signal transmission, it will be brought to a predetermined Aoperating frequency, even though its initial Afrequency may be widely at variance therefrom. 1,

It is an object of this invention, therefore, to

. provide an improved `frequencyresponsive network which avoids one or more of the disadvantages of the prior-art arrangements and which has an improved sensitivity.

It is a further object of this'invention to provide a frequency-responsive network operablev over a relatively narrow frequency range having a relatively high sensitivity over the range and producing relatively large control effects.

In accordance with the invention, the frequency-responsive network operable over a given frequency range comprises means for developing from an applied signal a first voltage which decreases in amplitude with frequency in the vicinity of the mean frequency of the range and means for developing from the applied signal a second voltage which increases in amplitude with frequency in the vicinity of the mean frequency of the fange. The network includes a rectiers, each of which has an output circuit, and means for applying one of the developed voltages to each of the rectiers. The network further includes means for deriving a delay bias for each rectier solely from the output circuit of the other and means for deriving from the output circuits of the rectllers a control voltage which varies in accordance with the frequency of the applied signal. The term delay bias as used in the specification is intended to mean an amplitude delay bias. l For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description'taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. v

In the drawing, Fig. 1 is acircuit diagram. partly schematic, of a complete superheterodyne receiver including a frequency-responsive network embodying the present invention; Fig. 2 is a graph representing certain operating characteristics of the circuit of Fig. l; and Fig. 3 is a circuit diagram of a modification of the arrangement of Fig. 1.

Referring to Fig. 1 of the drawing. there is shown a circuit diagram, partially schematic, of a complete modulated-carrier signal receiver of the superheterodyne type embodying the invention. This receiver comprises, in cascade, lan antenne-ground circuit I0. il, a radio-frequency amplifier I2. a frequency changerfor oscillator.- modulator i3, an intermediate-frequency ampliner I4 which may comprise. one or more'stages of amplification. a further vstage including a A vacuum-tube amplifier lli, a frequencyrespon` ysive. network It constructed vin accordance with pair of the invention as will be pointed out more fully hereinafter, a detector andl automatic amplication control or A. V. C. supply I1, an audio-frequency amplier I8 which may comprise any desired number of stages, and a sound reproducer i9. Automatic amplification control is obtained in a well-known manner by a unidirectional bias voltage derived from the A. V. C. supply I1 and applied over conductor 2l to the control electrode of one or'more of the tubes included in the radio-frequency amplifier I2, oscillator-modulator 3 and intermediate-frequency amplifier Iii.

. Automatic frequency control issecured in a wellknown manner by applying a unidirectional voltage derived from the frequency-responsive network I5, in a manner to be described in detail hereinafter, over a conductor 22 to a frequency-adjusting device 23 which, in turn, is coupled to the oscillator-modulator I3 by way of a conductor 24. All of the components of the superheterodyne receiver with the exception of the frequency-responsive network I6, may be of any suitable conventional construction.

Neglecting for the moment the operation of the frequency-selective network I6, the circuit above described comprises, in general, a conventional superheterodyne receiver including automatic volume control and automatic frequency control systems.- The operation of such a receiver is well understood in the art so that a detailed description thereof is unnecessary. In brief, however, a signal intercepted by the antenna-ground circuit I, I I is selected and amplified in the radiofrequency amplifier I2 and applied to the oscillator-modulator I3, wherein it is converted into an intermediate-frequency signal. The intermediate-frequency signal is amplified in the intermediate-frequency ampliers I 4 and I5 and is supplied to the detector I1, wherein the audiofrequency signal components and the A. V. C. biasing potential is derived. The audio-frequency signal components, in turn, are applied to the audio-frequency amplifier I8, wherein they are amplified and delivered to the sound reproducer I9 for reproduction. The automatic amp1ifica tion control bias derived from the A. V. C. supply I1 is effective to control the amplication of one, or more of the units I2, I3 and II to maintain the signal input to the detector I1 within a relatively narrow range for a wide range of received signal intensities, while the frequency-responsive network I 6 is effective to supply an automatic frequency control bias to the frequency-adjusting device 23 to maintain the carrier frequency of the signal input to the detector` I1 at a substantially constant value.

Referring now more particularly to the circuit embodying the present invention, for the purpose of developing automatic frequency control bias to govern the tuning of the receiver, the frequency-responsive network comprises means for developing from an applied signal a first voltage which decreases in amplitude with frequency in the vicinity of the mean frequency of the operating range of the network and means for'developing from the applied signal a second voltage which increases in amplitude with frequency in the vicinity of the mean frequency of the range. Specically, these means comprise a primary tuned circu-it including condenser 30 and induen tor 3| located in the output circuit of the vacuum tube amplifier I5, a secondary tuned circuit including condenser 32 and center-tapped inductor 33. and a coupling condenser 34 interconnecting the high-potential side of the primary tuned circuit and the mid-tap of the secondary winding 33. The tuned circuits are inductively coupled to one another and are tuned to the normal intermediate frequency of the receiver which corresponds to the mean frequency of the operating range of network I6. An inductor 35 is also inductively coupled to the secondary winding 33 in order to derive the input signal for the detector I1. A pair of triode rectiers 36 and 31 is included in the network, each of which contains an anode, a' cathode and a control electrode, and means are provided for individually .applying the aforementioned developed voltages to the rectifirs, no other source of anode potential being employed. To this end the rectiers are connected tc opposite terminals of the secondary tuned circuits, the cathodes of rectiers 3S and 31 being interconnected through secondary winding 33, which has a relatively low direct current resistance so that the cathodes are maintained at substantially the same unidirectional potential.

The control electrode of each rectifier, in turn, is connected to and maintained at substantially the same unidirectional potential as the anode of the other rectifier, whereby substantially the entire output voltage oi. each rectier is applied as a delay bias to the control electrode of the other. Resistors 38 and 39 are included in circuit with the control electrodes of rectiflers 36 and 31, respectively, and, if desired, the control electrode of each rectifier may be by-passed to its cathode through a condenser, as shown, in orderto prevent grid rectification. Output circuits which contain no common elements are provided for the rectiflers. The output circuit of rectifier 36 includes a load resistor 4D, while that of rectifier 31 includes a load resistor 4I.

A control voltage, which varies in accordance with the frequency of the applied signal, is derived from the output circuit of the network which includes load resistors 40 and 4I connected in series by the secondary winding 33 and applied to Va tuning indicator or milliammeter 4I, bypassed by condenser 43,and included in the output circuit of the network. The control voltage is also applied over conductor 22 to the frequency-adjusting device 23 for the purpose of controlling the tuning of the receiver.

In considering the operation of the circuit just described, itwill be seen that it is similar to conventional frequency-responsive networks in that it employs the principle of supplying separately the sum and difference of the voltages developed across the primary and secondary of a doubletuned circuit to the individual ones of a pair of rectiers for rectification to obtain a voltage varying in magnitude and polarity with the frequency ofthe applied signal. The circuit differs from conventional frequency-responsive networks, however, in that it supplies as a delay bias to each rectifier a bias derived solely from the output circuit of the other and which, in this embodiment of the invention, is substantially the entire output voltage of the other, which bias is eective to reduce the output of the rectiner to which it is applied substantially to zero at a frequency in the-vicinity of the mean frequency of the range. This may be more clearly understood by considering the operation o1' the network under speciiic operating conditions.

Assume that the frequency oi' the appliedsig The following circuit constants were used inl cuits so that the control voltage derived from l the network is zero. Assume now that the vfrequency of the applied signal increases to a value above the mean frequency of the range. It will be seen that the output of one of the rectifiers,

for example rectifier 36, increases, while that of the other, rectifier 31, decreases. Further, the delay bias applied to the control electrode of rectifier 31, which comprises substantially the entire output voltage of rectifier 36 due to the stantlally to zero at a frequency very near the mean frequency of the range.

Hence, when the frequency ofthe applied signal increases beyond this cutoff frequency of rectifier 31, the control voltage derived from the network is substantially equal to the output voltage of rectifier 36. Conversely, when the frequency of the applied signal is decreased to some value below but very near the mean frequency of the range, the delay bias derived from rectifier 31 is effective to bias rectifier 36 beyond cutoff, whereby the control voltage derived from the network is substantially equal to the output voltage of rectifier 31.

The characteristic discriminator S curve of the circuit of Fig. 1 is represented by curve A of Fig. 2, while curve B represents the characteristic discriminator S curve for conventional frequency-responsive networks. The slope of the vdiscriminator characteristic of the circuit of Fig. l is seen to'be very steep near the mean carrier frequency, as compared with that of conventional frequency-responsive networks, which demonstrates that the frequency-responsive network of the invention is much more sensitive than conventional arrangements. The steepness of this slope and, consequently, the sensitivity of the network, is very great in the vicinity of the mean.

carrier frequency because the peaks of the characteristic are'relatively higher and are relatively tional frequency-responsive networks. This is due to the fact that the delay bias appliedto each rectifier is substantially the maximum bias voltage available and is applied to a control electrode so that it has a maximum biasing eiect and thereby may reduce the output of the rectifier to which it is applied to zero at a frequency very near the meen frequency of the range. It may also be seen from Fig. 2 that the control voltage derived from the output circuit of the system for the purpose of controlling the tuning of the receiver is considerably greater than that which obtaining the characteristic -curve A and are illus.- trative of a specificapplication of the invention:

Mean frequency kilocycles-- 1000 Rectiflers 36 and 31 -GT 6J5 R40 and R41 ....megOhmS 1 Ras and R39 ..-do. l 2.2 Condenser 43 micro-microfarads-- 150 A; modification of the tuning-indicator circuit of Fig. l and embodying this invention is shown in Fig. 3. AThe circuit of Fig. 3 may readily be employed in the receiver of Fig. 1 by connecting the terminal indicated as A with the corresponding terminal of Fig.` 1. The modifiedv form of tuning indicator is similar to that of Fig. 1. and corresponding circuit elements' are indicated by identical reference numerals. InFig. the anodes of rectifiers 36 and 31 are connected to the opposite sides of the .secondary tuned circuit by way of couplingcondensers 45 and 48, respectively, While the cathodes of the rectifiers are maintained at thev same unidirectional potential. As

I' in the arrangement of Fig. 1, the frequencycuit of rectifier 36 from which a delay bias is derived and applied to the control electrode of rectifier.31 and, in 'a similar manner, a delay bias derived from a voltage-divider resistor 43 ini cluded in the output circuit of rectifier 31 is applied to the control electrode of rectifier 36. The control electrodes are by-passed to their respective -,cathodes by way of condensers 50, 5I to prevent `rectification in the grid circuits. Resistors 52 and 53 included in the output circuits of the rectifiers and voltage dividers 41 and 48 `.comprise the arms of a bridge circuit including a tuning indicator 42. lThe bridge circuitis included in the output circuit of the frequency-responsive network and a voltage is derivedtherefrom which varies in polarity and magnitude in accordance with the frequency of the applied signal and is may be derived from conventional frequencyresponsive networks. The peaks of curve A are relatively broad, which shows that a relatively large control effect is derived from the frequency-responsive network of Fig. 1 for relatively large deviations of the frequency of the applied signal from the mean `frequency of the range, so that the network has a relatively large pull-in eiect. l

Thus, it may be seen that the arrangement of Fig. 1 is a very sensitive frequency-responsive network effective to derive a control bias supindicated by the tuning indicator 42 as a representation of the tuning of the receiver.

The operation of the circuit of Fig. 3 will be readily understood from the aforedescribed operation of the circuit of Fig. l. The delay bias applied to the control. electrode of each rectifier in the circuit arrangement of Fig. 3 is derived solely from means included in the output circuit of the other rectifier and comprises a part of the output volt-age of the rectifier circuit in which it is derived. whereas in the circuit of Fig. 1 the delay bias applied to each rectifier is substantially equal to the entire output voltage of the other rectifier. Further. since, in the circuit of Fig. 3, the delay bias is derived from a voltage-divider, the magnitude of the delay bias applied to the control electrode of each rectifier may be varied and, accordingly, the sensitivity of the frequency-indicaing network may be adjusted as desired.

Although the above-described embcaliments disclose lthe invention as applied to a. receiver, it

is to be understood that the invention may also be employed in connection with the frequency control of transmitters. When utilized in association with a transmitter, the large pull-in effect of the' frequency-responsive network is of particular value in bringing the oscillator to a preand modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A frequency-responsive network for operation over a given frequency range comprising, means for developing from an applied signal a first voltage which decreases in amplitude with frequency in the vicinity of the mean frequency of said range, means for developing from said applied signal 'a second voltage which increases in amplitude with frequency in the vicinity of the mean frequency "of said range, a pair of rectiers, an outputcircuit for each of said rectifiers, means for individually applying said voltages to said rectii'lers, means for deriving a delay bias for each of said rectiiiers solely from the output circuit of the other, and means for deriving from the output circuits of said rectiers a control voltage which varies in accordance with the frequency of the applied signal.

2. A frequency-responsive network for operation over a given frequency range comprising, means for developing from an applied signal a first voltage which decreases in amplitude with frequency in the vicinity of the mean frequency of said range, means for developing from said applied signal a second voltage which increases in amplitude with frequency in the vicinity of the mean frequency of said range, a pair of rectifiers, output circuits for said rectiiers having no common elements, means for individually applying said voltages to said rectiflers, means for deriving a delay bias for each of said rectiers solely from the output circuit of the other rectifier, each of said biases being sumcient to reduce the output of the rectier to which it is applied to zero at frequency in the vicinity ofthe mean 'frequency of said range, and means for deriving from the output circuits of said rectifier-s a control voltage which varies in accordance with the frequency of the applied signal.

3. A frequency-responsive network for operation over a given frequencyrange comprising. means for developing from an applied signal a rst voltage which decreases in amplitude with frequency in the vicinity of the mean frequency,

of said range, means fordeveloplng from said applied signal a second voltage which increases in amplitude -with frequency in the vicinity of the mean frequency of` said range, a pair of rectifiers each of which has a control electrode, an output circuit for each of said rectiers, means for individually applying said voltages to said rectiiiers, means for deriving a voltage lsolely from the output circuit of each rectier, means for applying the voltage derived from the output circuit oi' each of said rectifiers as a delay bias to the control electrode ofthe other, and means for deriving from the output circuits of said rectifiers a control voltage which variesin accordance with the frequency of the applied signal.

4. A frequency-responsive network for operation over a given frequency range comprising, means for developing from an applied signal a first voltage which decreases in amplitude with frequency in the vicinity of the mean frequency of said range, means for developing from said applied-signal a second voltage which increases in amplitude with frequency in the vicinity of the mean frequency of said range. a pair of rectilers, an output circuit for each of said rectiers, a pair of voltage dividers included individually in said output circiuts, means for individually applying'said voltages to said rectiers, means for deriving a delay bias for each of said rectiiiers from the voltage divider included in the output circuit of the other, and means for deriving from the output circuit of said rectiilers a control voltage which varies in accordance with the frequency of the applied signal.

5. A frequency-responsive network for operation over a given frequency range comprising,

means for developing from an applied signal al 2o rst voltage which decreases in amplitude with frequency in the vicinity of the mean frequency of said range, means for developing from said applied signal a second voltage which increases in amplitude with frequency in the vicinity of the mean frequency of said range, a pair of rectiiiers, output circuits for said rectiers having no parts in common, means for individually applying said voltages to said rectiers, means for applying substantially the entire output voltage of each of said rectiiiers as a delay bias to the other, and means'for deriving from the output circuits of said rectiiers a control voltage which varies in accordance with the frequency of the applied signal.

6. A frequency-responsive network for operation over a given frequency range comprising, an input circuit, means coupled to said input circuit for deriving a rst voltage, means coupled to said input circuit for deriving a second voltage 4Q variable in phase with frequency over said range with respect to said rst voltage, a pair of rectifiers, output circuits for said rectiers, means for applying to one of said rectiflers the sum .and to the other Aof said rectiers the difference of said two voltages, means for deriving a delay bias for each of said rectiers solely from the output circuit of the other, and means for dei riving from the output circuits of said rectiiiers a control voltage which varies in accordance with the frequency of the applied signal. 7. A frequency-responsive network for operation over a` given frequency range comprising, means for developing from an applied signal a first voltage which decreases in amplitude with frequency in the vicinity of the mean frequency of said range, means for developing fromsaid applied signal a second voltage which increases in amplitude with frequency in the vicinity of the mean frequency of said range, a'pair of rectifiers each of which includes an anode, a cathode and a control electrode. said cathodes being maintained at substantially the same unidirectional potential and the control electrode of each rectifier being maintained at substantially the same unidirectional potential as the anode of the other rectifier, means for individually applying said voltages to said rectiiiers whereby substantially the entire output of each rectifier is applied as a delay bias to the control electrode of the other rectier, and means for deriving from the output circuits of said rectifiers a control voltage which varies in accordance with the frequency of the applied signal.

8. A frequency responsive network for opera- ?5 tion over a given frequency range comprising, a

pair of coupled tuned circuits resonant at the mean frequency of the range for developing from an applied signal a first voltage and a second voltage variable in phase with frequency over said range with respect to said rst voltage. a pair of rectiers, output circuits for said rectiilers, means for applying to one of said rectiers the sum and to the other or said rectiers the dinerence of said two voltages. means for derivins a' msnm F. 

